1. Field of the Invention
This invention generally relates to methods for the electrostatic treatment of water streams. In particular, the invention describes a method for preventing the formation of deposits, especially biofilm deposits, on conventional membrane-separation systems by the application of a capacitive electrostatic device.
2. Description of the Related Art
Water treatment systems are commonly subject to reduced efficiency and failure as a result of scaling and clogging by solid particles suspended in the aqueous medium, as well as by the formation of biofilms that adhere to exposed surfaces in the equipment. Corrosion caused by biofilms, biosludge and scale deposits is also a source of equipment failure and maintenance challenges. The concept of treating the water by inducing an electrostatic field across it has been known and many devices have been utilized with varying success, both for industrial and domestic applications, as a means for causing the precipitation and settling out of mineral and organic deposits. To the extent that prior-art system have been successful in facilitating the removal of suspended material, they still have not prevented the formation of mineral and organic deposits, especially biofilms, on filtration membranes and wetted surfaces of water treatment plants.
Capacitive electrostatic fields are created in a body of flowing water by an insulated electrode arranged to produce a capacitive layer across the water. The integrity and strength of the insulation between the water and the electrode is crucial for the continued operation of a system because any breakdown of the dielectric layer causes a current leakage or short through the water body and the inevitable shutdown of the system. Therefore, capacitive electrostatic devices must be constructed such as to ensure the integrity of the dielectric material used to insulate the electrode (which is usually positive for scale-reduction applications). This has been achieved in the art by encasing a tubular metallic electrode in a Teflon® sleeve which is heat shrunk around the outer surface of the electrode, and by sealing each end of the resulting insulated electrode with protective dielectric bushings. A seamless insulating layer of Teflon® around the metallic electrode has thus been used to ensure intimate contact between the two materials. Such intimate contact is very important because any air space left between the metal and the dielectric, such as produced by blisters or bubbles in the dielectric layer, causes electrical arcing that eventually perforates the Teflon® layer, shorts the electrode to the water body, and greatly reduces the electrostatic efficiency of the device. Moreover, a large air space would form yet another dielectric layer within the system, which is undesirable because of the very low capacitance of air which would further reduce the overall capacitance of the system.
Because the capacitive electrostatic field across a water medium is proportional to the potential applied to the system, it is desirable to apply as high a voltage as possible within the tolerances of the apparatus. As I disclosed in U.S. Pat. Nos. 5,817,224 and 5,591,317, higher voltages have been found to be more effective, at times essential, for treating waters with high dissolved or suspended solid concentrations (such as with more than 1,000 ppm total dissolved solids) which have been shown to be totally unaffected by conventional devices and methods that can only operate at less than 10,000 volts. For a given water quality and flow rate, there is a critical field intensity below which no capacitive electrostatic effect is noted.
The devices of the prior art are limited in their application by twofold problems. Because of its well-known physical properties, PTFE material such as Teflon® is not suitable for adherence to the surface of metal conductors other than by heat-shrink processes. Any attempt to cover an electrode with Teflon® by a process other than heat-shrinking, such as would be required for an electrode having a non-cylindrical shape, would necessarily compel the formation of seams and connections which would be very difficult to achieve and prone to breakdown during use. In addition, due to the non-stick properties of the material, it would be very difficult to avoid the formation of air spaces between the metal and the dielectric surfaces. Accordingly, the preferred structure of such electrostatic devices is cylindrical with each end sealed by means of separate dielectric bushings. Under normal stresses of operation, the connection between the tube and these end bushings has been the source of leaks which allow the water medium to come into contact with the high-voltage metallic tube and cause a complete system breakdown.
Another problem relates to the thickness of the dielectric material utilized in the prior art. In order to optimize capacitance, the layer of Teflon® used to coat the positive electrode is kept to a minimum, thereby causing the dielectric layer to be more vulnerable to imperfections of construction which can cause arcing or other operating stresses that could result in interruption of insulation. As a result of these constraints, the devices of the prior art are not suitable for efficient and dependable operation at voltages higher than approximately 10,000 volts, beyond which they quickly experience breakdowns. This characteristic has prevented their utilization for large water-treatment systems and for waters containing high concentrations of dissolved solids, both of which require very high electrostatic potentials applied across the water body in order to process high-volume throughputs.
Because of these practical problems, the concept of applying an electrostatic field to a water suspension to effect its physical characteristics has been exploited only in relatively small water treatment systems having low throughput and/or low solid content, and only in an effort to reduce scaling and remove fine particulate matter. In U.S. Pat. No. 5,591,317, I disclosed a new electrostatic device which is operable at very high voltages with reliability and safety. In particular, I demonstrated that such device is not susceptible to total breakdown as a result of breakage or interruptions in the dielectric integrity of the material. Given the relatively high voltage at which my capacitive electrostatic device can be safely and reliably operated, I have explored its use for improving other processes such as chemical flocculation, disclosed in my application Ser. No. 09/167,115, and now membrane separation, particularly reverse osmosis (RO), which are important processes utilized in the treatment of water for public or industrial use.
The major problem encountered in membrane separation plants (reverse osmosis, nanofiltration, ultrafiltration) is the fouling of membranes caused by mineral, organic and biological deposits. These deposits affect system performance because clean-water recovery and quality necessarily decline as membrane fouling increases. In addition, the bacteria and other microorganisms entrapped on the membrane's surface cause them to become plugged, which in turn requires increased operating pressures and energy consumption, and greater frequency of chemical washing, all of which results in irreversible equipment damage.
Thus, this disclosure is directed to improving conventional membrane-separation processes by the application of capacitive electrostatic fields produced by the high voltages permitted by devices such as disclosed in U.S. Pat. No. 5,591,317. The invention is based on the discovery that such high-voltage induced fields essentially prevent biofouling of membranes and produce greater permeate recoveries than possible in the prior art. A number of prior inventions, such as disclosed in U.S. Pat. Nos. 3,933,606, 4,238,326, 4,755,305, 4,802,991, and 4,915,846, have utilized an electric power source to improve water purification and dewatering processes. Others, such as described in U.S. Pat. Nos. 4,024,047, 4,902,390, and 5,326,446, have used electrostatic and electromagnetic fields to purify waters of biological material and bacterial contaminants by reducing their propagation and causing them to settle out. In particular, U.S. Pat. No. 4,886,593 taught a method for killing or inhibiting the growth of bacteria in water by subjecting the water to an electrostatic field of sufficient intensity to produce that effect, preferably in the presence of a leakage current in the order of several milliamps.
The preference for the presence of a leakage current in the order of milliamps is consistent with historical findings in the art of killing bacteria with electrical phenomena. Since the middle of the last century, the application of electrical currents has been reported to kill bacteria. The bactericidal mechanism was postulated to be the induction of mutations, or by some impact of the charge and subsequent cavitation of the bacterial organisms. In 1967, DC pulses up to 25 kv/cm were tested on bacterial suspensions and found to kill a number of bacteria, but as a result of thermal effects rather than electrolysis. The current density ranged from 8 to 61 amps/cm2. See A. J. Sale, “Effects of High Electric Fields on Microorganisms,” Biochimica et Biophysica ACTA 781-788 (1967). In 1988, a method of reversible breakdown of lipid membranes was reported using current densities in the order of 1 amp/cm2 produced by the application of 0.5-1.9 volts. Cell membranes temporarily lost their barrier function by creating hydrophilic pores when exposed to these relatively high electrical potential differences. R. W. Glaser, “Reversible Electrical Breakdown of Lipid Bilayers Formation and Evolution of Pores,” Biochimica et Biophysica ACTA 275-286 (1988). In 1989, based on experiments with synthetic urine and iontophoresis carried out using 10-400 micro-amps of current, it was determined that the lethality of the current was directly related to the amperage. C. P. Davis, “Effects of Microamperage, Medium, and Bacterial Concentration on Iontophoretic Killing of Bacteria in Fluid,” Antimicrobial Agents and Chemotherapy 442-447 (1989). Follow-up work in 1991 showed that bacterial and fungal killing could be accomplished with iontophoretic technology and improved electrodes using up to 400 micro-amps of current. C. P. Davis, “Bacterial and Fungal Killing by Iontophoresis with Long-Lived Electrodes,” Antimicrobial Agents and Chemotherapy 2131-2134 (1991). Using low-intensity electrical fields of 12V/cm2 and low current strengths of 2.1 mA/cm2, Blenkinsopp demonstrated electrical enhancement of biocide efficiency against P. aeruginosa in biofilms in 1992. S. A. Blenkinsopp, “Electrical Enhancement of Biocide Efficacy Against Pseudomonas Aeruginosa Biofilms.” Applied and Environmental Microbiology 3770-3773 (1992). The mechanism was felt to be either electroporation, electrophoresis, or iontophoresis. In 1994, Davis used a 400 micro-amp current to convert chloride ions present in synthetic urine to chlorine-based substances and concluded that this was the basis for the antimicrobial effect of iontophoresis. C. P. Davis, “Quantification, Qualification, and Microbial Killing Efficiencies of Antimicrobial Chlorine-Based Substances,” Antimicrobial Agents and Chemotherapy 2768-2774 (1994). In 1995, currents of up to 20 mA/cm2 were used to demonstrate that they had no detrimental effect on biofilms, but confirmed that they enhanced the performance of tobramycin against P. aeruginosa. J. Jass, “The Effect of Electrical Currents and Tobramycin on Pseudomonas Aeruginosa Biofilms,” Journal of Industrial Microbiology 234-242 (1995). In 1996 Wellman reported an independent confirmation of this bioelectric effect with currents of 1-5 mA/cm2. N. Wellman, “Bacterial Biofilms and the Bioelectric Effect,” Antimicrobial Agents and Chemotherapy 2012-2014 (1996). Later that year, work with 9 mA/cm2 and antibiotics suggested that an electrical current can enhance the activity against biofilms of those antibiotics that are effective against planktonic cells. J. Jass, “The Efficacy of Antibiotics Enhanced by Electrical Currents Against Pseudomonas Aeruginosa Biofilms,” Journal of Antimicrobial Chemotherapy 987-1000 (1996).
Thus, after several decades of research with different levels of AC and DC voltages, experiments have shown that bacteria are most effectively eliminated by currents in the milliamp range produced by low DC voltages. No one has anticipated or suggested the use of a very-high-voltage capacitive electrostatic field alone, in the absence of measurable currents, as an effective means for reducing biofilm formation in water systems in general, nor in particular in filtration membranes where such reduction also results in a greater efficiency of permeate production.
All prior attempts at controlling the formation and growth of biofilms in the art of membrane water treatment are based on the use of chemical biocides and chemical dispersants, many of the latter having been used for some time in promoting the suspension of mineral particles. The effect of chemicals on the formation of biofilms, though, has been very limited, both in scope and duration, providing little relief to the pervasive and enduring problem of biofilm formation and related fouling of membrane-separation units.